In the field of aircraft ice protection, people usually call water droplets with a median volumetric diameter (MVD) of less than 50 microns normal-state water droplets, and call water droplets with a median volumetric diameter of above 50 microns large water droplets.
Abnormal-state ice caused by supercooled large droplets (SLD) always presents difficulty in aircraft ice protection and ice detection. This is because large water droplets have a larger mass than normal-state water droplets and therefore have a larger inertia and greater internal latent heat, so they are more likely to flow on the surface of the aircraft during icing and need to consume longer time. As such, when supercooled large water droplets contact a surface of an object, they will not, like normal-state water droplets, ice quickly at a contact portion or its vicinity, but move a certain distance on the surface of the object and then get iced. In this way, icing might happen at none-icing-protection positions of the aircraft such as at upper and lower airfoil surfaces after a front edge of the airfoil and a tail wing so as to form the so-called “backflow ice”. Such icing is more dangerous to the flying safety of the aircraft than the normal-state icing.
Currently, people have already developed several kinds of detectors for detecting supercooled large water droplets, for example, two patents with publication number US2002/0158768A1 and WO03/002410A1. The basic technical solution thereof is as follows: different flow channels or water droplet capturing flow fields are designed for normal-state water droplet and large water droplets, and two independent ice detectors are applied so that the ice of normal-state water droplets and the large water droplets is respectively frozen on the two ice detectors so as to identify and detect the abnormal-state icing of the large water droplets. Such device is capable of achieving identification and detection of large water droplets, but it is disadvantageous in a complicated detector structure, a large size and difficulty in processing.
In addition, regarding normal-state water droplets with a median volumetric diameter of less than 50 microns, various current ice detectors still have drawbacks hard to overcome in terms of icing detection. This is the reason why it is impossible to perform accurate judgment of types of icing (e.g., clear ice, rime ice or mixed ice) caused by the normal-state water droplets so that an error of a measurement of a thickness of an ice layer is caused when the thickness of the ice layer is detected. This is because ices with the same thickness but in different types require different detection electrical signals. Hence, the same electrical signal might indicate various icing conditions with different types and different thicknesses. As such, it is difficult to make an accurate judgment of the thickness of the ice after the detection signal is received.
Additionally, it is noticeable that the term “ice” mentioned in the present application shall include various types of ice, frost and mixtures thereof.